Steam has been used in decontamination systems, such as sterilizers or autoclaves located in hospitals, manufacturing settings, and laboratories to sterilize devices such as medical and dental instruments, laboratory instruments, production equipment, manufactured products, and other articles. Steam is also used for vulcanizing, cooking, melting, humidifying, and drying in process industries such as pharmaceuticals, foods, paper, automotive, and printing, etc.
Steam purity and steam quality are important properties of steam that will affect the efficacy of a decontamination process, such as steam sterilization. Steam purity is an expression of the quantity of non-water components (vaporous contamination) carried in the steam. Steam quality refers to the quantity of moisture present in the steam. If there is no moisture (i.e., no liquid water), then the steam is of 100% quality. Accordingly, “pure” steam has a liquid water content of 0%. It should be appreciated that steam quality relates to steam purity because liquid droplets in steam may contain dissolved solids that may be transmitted to the process.
In many healthcare and pharmaceutical applications, the minimum acceptable steam quality for a steam sterilizer or autoclave is 95%. If steam quality is below 95%, then “wet packs” (i.e., moisture droplets) may develop on articles after completion of a sterilization cycle. Consequently, reprocessing will be required, and/or batches of product may have to be discarded.
Aside from decontamination, many processes require steam that is dry and contains no superheat. Printing presses use steam to control static electricity and to precisely control the drying and shrinkage of the paper. Poor steam quality can upset the control. Wet steam can even cause the paper to tear, ruining a press run. Dairy applications inject steam directly into milk during pasteurization. Wet steam can carry contaminants and add too much water during the injection process.
A steam generator used to vaporize water can introduce contaminants into the steam, thereby reducing steam purity. For example, where the steam generator is a boiler, boiler chemicals can be introduced into the steam during priming or foaming of the boiler. These contaminants may cause corrosion or staining of the product or decontamination device (e.g., steam sterilizer) or articles to be processed by exposure to the steam.
Attempts have been made to check the quality of steam in an effort to reduce some of the problems caused by steam with an inappropriate quality. U.S. Pat. No. 4,561,785 issued to Long claims to disclose a “method and apparatus for determining the quality of typical steam used in steam flooding for secondary recovery of petroleum” by continuously leaking a portion of high-pressure steam out of the system. Although leaking hot steam out of a system may be acceptable for oil well fields, there are many situations where such a hazard is not acceptable.
Other methods of analysis have been attempted such as in U.S. Pat. No. 4,547,078 issued to Long where steam quality is measured at a specific point in time by “obtaining a sample of the liquid component of steam and determining the quality of steam in a vessel or the like, such as steam flowing in a line used for steam injection in an oil well. The steam quality is determined by the known method of comparing the concentrations of dissolved solids in the liquid sample and the feedwater.”
These steam quality measurements are time consuming, inaccurate, and can expose operators to potentially unsafe conditions including scalding heat and deafening noise. Moreover, other approaches to measuring steam quality do not provide advanced warnings of problems with quality of the steam used in a process.